A rare disorder affecting the ear, otosclerosis causes abnormal bone growth in the bony labyrinth of the ear, affecting the ear ossicles, restricting their free movement, and impeding the transmission of sound to the inner ear. It is also called otospongiosis because the active phase is characterized by “spongy” bone formation, while the “sclerotic” tissue is seen in late stages. To understand the disease process, let us first discuss how the human ear functions.

The Human Ear

The human ear is the sense organ responsible for hearing. Functionally, it is responsible for the conversion of the sound waves into an electric impulse which is interpreted by the brain. There are three main compartments in the ear, each with an important task:

  • The outer ear, from the auricle to tympanic membrane, amplifies the sound waves by collecting them using auricle (or pinna) and conducting them to the eardrum (tympanic membrane).
  • The middle ear contains the three ear ossicles (small bones), namely malleus, incus and stapes, that connect the tympanic membrane to the cochlea, through the oval window. Their job is to convert the sound waves into mechanical energy in the form of physical vibrations and transmit this energy to the inner ear.
  • The inner ear (otic capsule) includes mainly the cochlea (the hearing organ) and three semicircular canals (the balance organs). The mechanical energy from the ossicles is transmitted to fluid-filled cochlea by the stapes, which leads to the generation of an electric impulse by small hair-like cells, which is carried to the brain by the eighth cranial nerve (vestibulocochlear).

Accordingly, various diseases affect different compartments of the ear, such as otitis externa (swimmer’s ear), foreign body impaction, furunculosis, tympanic rupture in the outer ear, otitis media (acute and chronic), otosclerosis, cholesteatoma, mastoiditis in the middle ear, and Meniere’s disease, acoustic neuroma, tinnitus, vertigo in the inner ear. Each of these requires a different management approach.

Hearing loss is one of the most important symptoms related to ear diseases, and is of two types; conductive or sensorineural hearing loss, depending on which part of the transmission pathway is defective.

Understanding Otosclerosis

1704 Valsalva described Ankylosis of stapes (stapes fixation)
1857 Toynbee found that stapes fixation leads to hearing loss
1893 Adam Politzer described the clinical entity and called it “Otosclerosis”
1912 Siebenmann coined the term “Otospongiosis”

 

In otosclerosis, the most common ossicle affected is stapes, but the disease can also involve malleus, incus and/or the otic capsule. The main clinical finding in otosclerosis is hearing loss, predominantly of a conductive type, however, sensorineural or ‘mixed’ type of hearing loss is observed in a few cases.

Otosclerosis is a rare disorder affecting only about 0.3% of the population. The diagnosis is usually made in early adult life, and it has been found that the prevalence rises almost seven times with age (Niedermeyer et al, 2002). Epidemiologically, it has been found that otosclerosis is more common in Caucasians and certain Asian subpopulations, with about 10% of the white population being affected. Certain noticeable factors associated with the disease:

  • Commonly diagnosed between 20 to 40 years of life
  • Female to male ratio is 2:1
  • There is strong familial predisposition
  • Hormonal influences, especially estrogen, like pregnancy, menstruation and menopause adversely affect the disease
  • Stress in any form (trauma, surgery, etc.) aggravates the disease

The Cause and Effects

Otosclerosis is a genetically inherited disease with an autosomal dominant pattern of inheritance, but variable penetrance, meaning not every person carrying the abnormal gene shall have the disease. Recent studies have found out that certain viruses, such as the measles virus, affect the activation or progression of otosclerosis in some patients, leading to the hypothesis that specific genetic and environmental factors might act collectively to trigger the disease in an individual (Niedermeyer et al, 2007).

The pathologic lesion in otosclerosis is the formation of spongiotic or sclerotic bone tissue that replaces the normal bone tissue in the bony labyrinth of the ear.

  • In 80-90% of cases, the lesion is localized around the oval window and causes calcification of the annular ligament or leads to stapes fixation. In both these conditions, there is a conductive hearing loss (stapedial otosclerosis).The following figure shows the affection of stapes in otosclerosis.

  • Out of the remaining 10% cases, about 8% have cochlear involvement, thus presenting with sensorineural hearing loss (cochlear otosclerosis), and 2% have both cochlear and ossicular involvement, and develop the mixed type of hearing loss (stapedialand cochlear).

The Presentation & Diagnosis

The essential complaint that the patient presents with is hearing loss, usually appearing around the third decade of life, and mostly affecting the female. Following points in the patient’s history suggest otosclerosis:

  • Complaint of progressive loss of hearing over a few years, usually bilateral
  • History of hearing loss in the family, particularly one of the parents or sibling
  • Hearing loss worsened after trauma or surgery, and in women, after pregnancy or estrogen therapy.

Typical findings observed on a physical examination include:

  • conductive hearing loss in most of cases with bone conduction being better than air conduction (using Tuning fork tests)
  • low-volume speech, as they perceive their own voice louder because of profound bone conduction
  • Schwartze’s sign; otoscope examination shows a reddish-blue hue over the promontory and oval window regions.

Diagnosis of otosclerosis is guided by the following tests:

  • Pure tone audiometry usually demonstrates a low-frequency conductive loss in the early phase of otosclerosis. Gradually, the high-frequency loss appears with increased ‘air-bone’ gap. Carhart notch is a characteristic artifact seen on a PTA in otosclerosis with stapedial fixation.
  • Tympanometry reveals type A or As tympanogram, depicting stiffening of the ossicular chain.
  • Acoustic reflexes are abnormal and may be entirely absent in the later stages of the diseases.
  • Imaging studies like CT scan of the temporal bone may be useful in localizing the site of lesion for the purpose of surgical repair.

Management

Otosclerosis is a progressive condition but can be managed efficiently with the help of medical or surgical interventions, and amplification.

    1. Medical therapy:Daily sodium fluoridesupplementation has been found to arrest the progression of disease in most patients. Studies have shown that patients treated with fluoride supplements tend to have smaller otosclerotic lesions on temporal bone scans (Naumann et al 2005).
    2. Surgical intervention:Surgery is considered a cure for otosclerosis, but the hearing loss may recur in some patients. The treatment of choice for stapedial otosclerosis is stapedectomy, where the diseased stapes is removed and replaced by a prosthetic implant. A newer technique used these days is stapedotomy, where they drill a hole in stapes footplate and put in an implant, without complete removal of stapes. However, surgery can’t be used in patients with cochlear involvement or with mixed hearing loss.

  1. Amplification:Using hearing aids does not cure the disease; neither does it stop its progression. But in patients where surgery is not an option or where the disease has not progressed, various types of hearing aids can be used to manage the condition. Patients with good pre-operative hearing levels may also benefit by using cochlear implants that magnify the sound by bypassing the ossicles and directly stimulating the cochlea. However, use of such devices in otosclerosis only serves a palliative role, and without a surgical intervention, deafness is likely to ensue eventually.

Prognosis

Otosclerosis is a progressive and debilitating disease but not fatal. It adversely affects the quality of life of the patients and limits their day-to-day activities. However, the prognosis after surgery is very good. Studies have shown that 90% of the surgical patients have improvement in their hearing loss following the procedure. Hearing aids and medical therapy can pre-operatively augment the treatment. With effective and timely management, most patients are able to achieve a near normal level of hearing.

Future Possibilities

The last few decades have marked an era of extensive research around otosclerosis and provided us with numerous valuable insights about the condition. However, there is still a lot of mystery to be unraveled. To date, the most awaited discovery is that of a method to determine the susceptibility of a person or a way to halt or reverse the progression of the otosclerosis.

References:

  1. Images reference:
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  2. Parving, Y. S. (1999, 01). Clinical Otosclerosis, Prevalence Estimates and Spontaneous Progress. Acta Oto-laryngologica,119(4), 468-472. doi:10.1080/00016489950181017
  3. Niedermeyer, H. P., & Arnold, W. (2002, 05). Etiopathogenesis of Otosclerosis. Orl,64(2), 114-119. doi:10.1159/000057789
  4. Niedermeyer, H., Gantumur, T., Neubert, W., & Arnold, W. (2007). Measles Virus and Otosclerosis. Otosclerosis and Stapes Surgery Advances in Oto-Rhino-Laryngology, 86-92. doi:10.1159/000098676
  5. Naumann, I. C., Porcellini, B., & Fisch, U. (2005, 09). Otosclerosis: Incidence of Positive Findings on High-Resolution Computed Tomography and Their Correlation to Audiological Test Data. Annals of Otology, Rhinology & Laryngology,114(9), 709-716. doi:10.1177/000348940511400910

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